.\" -*- nroff -*-
.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2007-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Neighbor_allgatherv 3 "Jun 10, 2020" "4.0.4" "Open MPI"
.SH NAME
\fBMPI_Neighbor_allgatherv, MPI_Ineighbor_allgatherv\fP \- Gathers and distributes data from and to all neighbors. Each process may contribute a different amount of data.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Neighbor_allgatherv(const void\fI *sendbuf\fP, int\fI sendcount\fP,
	MPI_Datatype\fI sendtype\fP, void\fI *recvbuf\fP, const int\fI recvcounts[]\fP,
	const int\fI displs[]\fP, MPI_Datatype\fI recvtype\fP, MPI_Comm\fI comm\fP)

int MPI_Ineighbor_allgatherv(const void\fI *sendbuf\fP, int\fI sendcount\fP,
	MPI_Datatype\fI sendtype\fP, void\fI *recvbuf\fP, const int\fI recvcounts[]\fP,
	const int\fI displs[]\fP, MPI_Datatype\fI recvtype\fP, MPI_Comm\fI comm\fP,
        MPI_Request \fI*request\fP)

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
sendbuf
Starting address of send buffer (choice).
.TP 1i
sendcount
Number of elements in send buffer (integer).
.TP 1i
sendtype
Datatype of send buffer elements (handle).
.TP 1i
recvcount
Integer array (of length group size) containing the number of elements that are received from each neighbor.
.TP 1i
displs
Integer array (of length group size). Entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from neighbor i.
.TP 1i
recvtype
Datatype of receive buffer elements (handle).
.TP 1i
comm
Communicator (handle).
.sp
.SH OUTPUT PARAMETERS
.ft R
.TP 1i
recvbuf
Address of receive buffer (choice).
.TP 1i
request
Request (handle, non-blocking only).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Neighbor_allgatherv is similar to MPI_Neighbor_allgather in that all processes gather data from all neighbors, except that each process can send a different amount of data. The block of data sent from the jth neighbor is received by every neighbor and placed in the jth block of the buffer. The neighbors and buffer layout is determined by the topology of \fIcomm\fP.
.I recvbuf.
.sp
The type signature associated with sendcount, sendtype, at process j must be equal to the type signature associated with the corresponding entry in \fIrecvcounts\fP on neighboring processes.

.sp
.SH NEIGHBOR ORDERING
For a distributed graph topology, created with MPI_Dist_graph_create, the sequence of neighbors
in the send and receive buffers at each process is defined as the sequence returned by MPI_Dist_graph_neighbors
for destinations and sources, respectively. For a general graph topology, created with MPI_Graph_create, the order of
neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by MPI_Graph_neighbors.
Note that general graph topologies should generally be replaced by the distributed graph topologies.

For a Cartesian topology, created with MPI_Cart_create, the sequence of neighbors in the send and receive
buffers at each process is defined by order of the dimensions, first the neighbor in the negative direction
and then in the positive direction with displacement 1. The numbers of sources and destinations in the
communication routines are 2*ndims with ndims defined in MPI_Cart_create. If a neighbor does not exist, i.e., at
the border of a Cartesian topology in the case of a non-periodic virtual grid dimension (i.e.,
periods[...]==false), then this neighbor is defined to be MPI_PROC_NULL.

If a neighbor in any of the functions is MPI_PROC_NULL, then the neighborhood collective communication behaves
like a point-to-point communication with MPI_PROC_NULL in this direction. That is, the buffer is still part of
the sequence of neighbors but it is neither communicated nor updated.

.SH NOTES
The MPI_IN_PLACE option for \fIsendbuf\fP is not meaningful for this operation.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler
may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
MPI_Neighbor_allgather
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create
